Help with coral receding ?

[mlab]

Hi everyone! I just got into the hobby a few months ago! I picked up an established tank that already had some coral and fish in it and have been adding new corals since then! I have a question about this coral I added about a month or so ago. A few days ago I noticed it was turning white on the bottom. I checked parameters today
Magnesium: 1260
Calcium: 540
Alkalinity : 8.2
I have the coral in a rock on the sand. Can anyone help with what I might be doing wrong?? Thank you!

 

[Tahoe61]

Are you testing nitrate and phosphate, and if so can you provide those values? Salinity as well.
Are you dosing anything?
Type of lighting and intensity?

 

[mlab]

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[mlab]

Are you testing nitrate and phosphate, and if so can you provide those values? Salinity as well.
Are you dosing anything?
Type of lighting and intensity?

Nitrate is 20.9 and phosphate is .06.
I’ve only been dosing for alkalinity since that tends to dip below 8 occasionally and I try to keep it around 8.5. I have an ecotech marine light set to 30% intensity on one of their templates. It runs for about 9 hours a day

 

[Lavey29]

Nitrate is 20.9 and phosphate is .06.
I’ve only been dosing for alkalinity since that tends to dip below 8 occasionally and I try to keep it around 8.5. I have an ecotech marine light set to 30% intensity on one of their templates. It runs for about 9 hours a day

30% is nothing for par

 

[Science/G]

Not sure about the light settings, but Nitrates and phosphates are Ok and magnesium is a bit low

 

[Shirak]

Magnesium test kits are often inaccurate so not sure I would worry too much about the Mg. If you don't have many stony corals or coralline algae and are using a quality salt and doing water changes it is potentially higher than your test kit says. If it is where the test says it is.. yes low side but don't think it would be the cause of the receding.

30% setting on the light however could be a big deal. Which light exactly and what size tank?